Studies are currently being done on the immune character of the chimeras constructed with non-manipulated bone marrow and on the possibility of passively transfering chimerism to normal, irradiated mice. The work done in the past 12 months aimed at improving the system for induction of chimerism across the H-2 barrier in H-2d, H-2b and H-2k mouse strains. Timing of bone marrow cell injection after irradiation, dosage of MRF and careful handling of the cells harvested from the bones of the donor mice are all important factors for achieving long lasting chimerism. Many estabished and individually checked chimeras have been obtained, and their immune character has been investigated. Studies of chimerism have revealed that it is not transferable and that it is not eradicable even when massive amounts of recipient-type immunocompetent cells are transferred passively into the chimeras. Thus, long-lasting chimerism obtained by using non-manipulated bone marrow containing its own normal cytotoxic T cells seems to offer a source of basic information on the cellular and humoral vehicles of allotolerance. In fact, an interesting phenomenon has been observed in the chimeras which shows the paradoxical capacity of the chimeras to reject skin grafts from the same recipient-type H-2 strain. This capacity is acquired 2 months after transplantation, and it depends on donor-type cells which progressively replace recipient-type cells in the chimeras. Thus, we can now use a new model for studying alloreactivity when allotolerance to donor-recipient T cells is established. AKR leukemia-prone mice are being used to study bone marrow transplantation from leukemia-free mice (C57BL/6 or BALB/c). It has been observed that syngeneic reconstitution of AKR mice does not prevent or delay leukemia onset. On the contrary, AKR mice transplanted with C57BL/6 marrow have not yet shown signs of leukemia. C57BL/6 mice have been injected with radiation leukemia virus and transplanted with BALB/c bone marrow to see whether onset of lymphosarcoma (LS) can be prevented by conferring H-2d character to H-2b, LS-prone mice. Transplantable tumors of the H-2b type grow unimpaired in the chimeras. This model is being used for evaluation of possible H-2 restriction of thymus-derived T cells in the process of tumor rejection. The results show that no H-2 restriction can be advocated for the growth of the tumor because transfer of immuno-competent cells from normal, donor-type mice into the LS or B16 melanotic melanoma chimeric mice does not inhibit or suppress the growth of the tumors.